Mineralogy and Geology. 111 
on slaty cleavage by describing the various distorted forms of certain 
species of shells in fissile rocks, showing that these forms depend on 
the positions of the shells with relation to the direction of cleavage. 
He observes that the same shells in rocks that are not fissile are not 
thus distorted ; and on a single slab or layer the various specimens are 
all distorted in the same direction. ‘This observation led him to throw 
together many species which he had before considered distinct. 
He illustrates the subject by figures of distorted forms of the Spirifer 
giganteus and Sp. disjunctus from Tintagel and South Petherwin, copies 
of which, reduced one-half, are here given. (We have collected to- 
her the several separate cuts of Mr. Sharpe for more convenient _ 
comparison. ) 
| Figure 1 to 4, Spirifer disjunctus ; 5 to 8, Spirifer giganteus. All reduced one- 
half, except fig. a which is reduced two-thirds. 
Fig. 1 he S. disjunctus of its proper form, and the following 
stcateehatlacots : vt The lines zz mark t 
flattened or narrowed in a direction perpendicular to the cleavage, and 
by the distorting force. Figure 3 is an example of a cast lying at an 
Figure 4 represents another 
of the shell is pressed under 
the other part and concealed, and at the same time the remainder is so 
f the hinge portion are nearly —. 
ig : 
